1. Field of the Invention
The invention relates to an etching system, and more particularly to an etching system providing enhanced uniformity of etching applied to a wafer.
2. Description of the Related Art
After application of a photolithography process, a wafer is subject to etching by plasma in an etching chamber. Poly lines are thus produced on the wafer.
Referring to FIG. 1, a conventional etching system 1 comprises an etching chamber 10, a top RF (radio frequency) electrode 20, a quartz disk 30, a bottom RF (radio frequency) electrode 40, a base 50, a gas input pipe 60, an exhaust pump 70, and a focus ring 80.
The top RF electrode 20 and quartz disk 30 are disposed above the etching chamber 10. The top RF electrode 20 is disposed on the quartz disk 30. Here, the quartz disk 30 provides airtight functions to the etching chamber 10.
The base 50 is disposed in the etching chamber 10 and comprises a gas channel 51 extending from the interior of the base 50 to the exterior thereof in the etching chamber 10.
The bottom RF electrode 40 is disposed under the etching chamber 10 and base 50 and opposite the top RF electrode 20.
The gas input pipe 60 is connected to the gas channel 51 of the base 50 from the exterior of the etching chamber 10, inputting etching process gas into the etching chamber 10.
The exhaust pump 70 is disposed outside the etching chamber 10 and connected to an exhaust port 11 thereof. The focus ring 80 is disposed on the base 50.
As shown in FIG. 1, a wafer W, which has been processed with photolithography, is positioned in the focus ring 80. Namely, a wafer W having photoresist formed thereon is positioned in the focus ring 80. Etching process gas is input into the gas channel 51 of the base 50 via the gas input pipe 60 and further uniformly input into the etching chamber 10 from the base 50. The top RF electrode 20 and bottom RF electrode 40 operate to incur a capacitive effect in the etching chamber 10. Here, the etching process gas in the etching chamber 10 is subject to an electrochemical reaction and thereby ionized, transforming into plasma. The wafer W is etched by the plasma and portions, without the photoresist, thereof are removed thereby. Poly lines are thus produced on the wafer W. In another aspect, the plasma and corroded substances are expelled to the exterior of the etching chamber 10 or etching system 1 through the exhaust port 11 by the exhaust pump 70.
Nevertheless, a drawback occurs during the aforementioned etching process. Although the focus ring 80 concentrates the plasma on the wafer W, non-uniform flow or density distribution of the plasma is generated thereon due to powerful exhaust operation of the exhaust pump 70. Namely, above the wafer W, the flow speed or density of the plasma near the exhaust port 11 exceeds that far from the exhaust port 11. Specifically, the plasma with higher flow speed or density excessively etches the wafer W, while the plasma with lower flowing speed or density insufficiently etches the wafer W. Thus, the difference in critical dimension between the poly lines is large. Here, the critical dimension refers to the width of a poly line. For example, the difference in the critical dimension of the poly lines between the edges (near the exhaust port 11) and center of the wafer W may reach 9 nm. Accordingly, after the etched wafer W is cut into several chips, obviously widely varied electrical characteristics or capabilities result from the large difference in critical dimension of the poly lines thereon, thus reducing reliability of the chips.
Hence, there is a need for an etching system improving uniformity of flow or density distribution of plasma, effectively reducing the difference in critical dimension between poly lines.